RobinC,
AFAIK, the LGV used synthetic piston washers.
Walther did use leather on some of the early LG51/53/55s, and from what I have read, they were prone to using up what they had - so you may end up with a sightly later rifle with a leather seal.
However, by the 1960s, when the LGV came out, I would be very surprised if they used anything other than synthetic piston seals. (However, you never know )
It seems most of the modern piston seal replacements are designed to use the 25 mm Weihrauch parachute piston seals.
(I have ordered one of these for my LG55.)
Have fun
Best regards
Russ